S A T U R N : Part 3

PERIOD of ROTATION

Prior to the visitations of spacecraft to Saturn, there remained many observational problems with ascertaining the true rotational period. This is certainly due to the lack of any long-term distinguished atmospheric features and mostly lack of the very little colour contrast available to visual observers.

It was in 1789 that William Herschel (1738-1822) saw some faint features moving across the rings, and so he figured out for the first time that the ring rotation was about 10 hours and 32 minutes. It was then assumed the planetary rotation was the same as the ring period. This estimation appears fairly good, as it also corresponds does roughly match the rotation of the atmosphere on the equator we do adopt today. Similarly in 1793, Herschel used another two features and found the slightly smaller period of 10h 16.0m.

This latter result is also interesting, as the period agrees with the 1876 value found by Asaph Hall (1829-1907) of the average equatorial planetary rotation of 10h 14m 23.8±2.30s, by using an active outburst of several white spots. Again in 1894, using his 16.5cm. (6.5-inch) reflector, British born Arthur Stanley Williams (1861-1938) determined 10h 12.6m for the equatorial zone and at 20° latitude, 10h 14.3m. Yet another spot was seen at +36° latitude in 1903 by William Frederick Denning and Edward Emerson Barnard (1887-1923), and both declared they found the mean period of 10h 38.4m — and clearly different than the 1876 spot observed by Hall. Another in 1910 was seen in southern latitudes at −36° by Rev. Theodore E.R. Philips of Ashtead, Surrey in England using his 31.1cm. (12.3-inch) refractor. He found periods ranged from about 10h 38.0m to 10h 38.5m. Both Hough and Denning also then stated similar results around the same time as Rev. Philips.

The next major determination was with the “Great White Spot” discovered by W. Hay in 1930. In longitude, this spot covered about one third to quarter of the Saturnian disk being about 5″ to 7″ (arcsec) across. Aligned at latitude +15°, both Wright and Rowland found the rotational period as 10h 14m 07s, by timing the edges of the white area on the observed planetary meridian.

One of the greatest advances in the Saturnian rotation was achieved by J.H. Moore in 1938. (See Ref.12) This was done using the Lick Observatory 36-inch refractor by visual and spectroscopic analysis during various ring crossings of features. Moore also measured the Doppler rotation velocities at various latitudes. It was first ascertained that atmospheric rotational acceleration occurred towards the equator, agreeing well with rates observed with earlier spot values.

During 1950 to 1960, observations of various features were found to change velocities significantly over three months or so. Modern values for the Saturnian rotation quote System III as 10.656 hours or 10h 39m 22s which surprisingly concurs well with Denning and Barnard!

Note: In 1990, the Hubble Space Telescope produced an animation showing that the general Saturnian atmospheric motion and cloud patterns.
See NASA Astronomy Picture of the Day.

Summary of the Period of Rotation

Central Meridian Systems for Saturn

The International Astronomical Union (IAU) originally decided to agreed on the rotation rate of Saturn in the mid-1960s. To save having the very complicated differential rotation rate, Saturn like Jupiter was simply divided into the visual phenomena of System I and System II. These values were based on visual observations of Saturn whose sole purpose was to identify and measure any new atmospheric phenomena for proper study. This was mostly applied to observed active features often seen within the NEB(S), EZ and SEB(N) zones.

In reality, as long as the standard rotational period is applied, then some surface feature relative to other surrounding belt makes little difference — even if the rotation period were slightly inaccurate. This also procedure was again applied directly to the features known as the Great Red Spot (GRS) on Jupiter. For example, the GRS drifts through its assigned latitude at different rates than the latitude it lies along. Times of transits are predictable via ephemerides.

When radiometric measures were made for Saturn, the resultant rotation period was found to be different than the earlier System I and II. Radio waves from Saturn are produced by the magnetic field produced by a metallic hydrogen region surrounding the small solid core. Named System III, it is presently preferred to describe Saturnian atmospheric features. Astronomers also now favour System III because it is likely reflecting the true rotation of the inner core and is not subject to atmospheric changes based on the latitude of the observed phenomena in question.

System III is used in all other regions and has the drift rate standardised at 10h 39m 22s. This produces 810.80236° per day. System III is generally rounded as the drift rate of 810° per day. [Based on eight times at 810° per day or four times 10h 39m 22s]

System I is used for the equatorial regions for the zones to about ±30°. Using the rotational ‘day ’ of 10h 14m produces 844.29967° per day. System I is generally rounded to have drift rates of 844° each day. [Based on three rotations times at 844° per day.]

Since late 1990s the IAU no longer supports nor gives information on any of the parameters for System I, and System III replaces the old visual System II. System III parameters are now exclusively supported by the IAU. [See Davies, M.E., et al. (1996)].

Rotation References

1. Barnard, E.E., “White Spot in Saturn”, AJ., 23, 180 (1903)
2. Campbell, W.W., AJ., 2, 416 (1895)
3. Cragg, T.A., “Rotation of Saturn”, PASP, 73, 314. (1961)
4. Denning, W.F., “Astronomical Publications.”, J.BAA., 14, 176 (1904)
5. Davies, M.E., “Report of the IAU/IAG/COSPAR Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 1994”, Celestial Mechanics, 63, p.127 (1996)
6. Gurnette, R.L., Woolley, R.v.d.R, “Explanatory Supplement to the Ephemeris ”,
   Pub. USNO (1961)
7. Hall, A., “On the Rotation of Saturn ”, A.N., 90, 145 (1877)
8. Hellad, R., Schubert, G., Anderson, J.D., “Jupiter and Saturn Rotation Period”,
   arvix:0907.3418v1, [astro-ph.EP] (20 Jul 2009) (2009)
9. Herschel, W., Phil.Trans.Roy.Soc., 84, 48 (1794)
10. Hough, G.W., “Rotation Period of Saturn”MNRAS, 64, 550 (1904)
11. Hough, G.W., “Rotation Period of Saturn”, MNRAS., 64, 122 (1903)
12. Read, P.L., P.L, Dowling, T.E., Schubert, G., “Saturn’s rotation period from its atmospheric planetary-wave configuration”; Nature, 460, pg.608 (Letter, 30 July 2009) (2009)
13. Moore, J.H., “ Spectroscopic Observations of the Rotation of Saturn”, PASP., 51, 274 (1939)
14. Pannekoek, A., “A History of Astronomy ”, Dover Pub. (1961)
15. Rowland, J.P., “The Rotation Period of the Equatorial Zone of Saturn, from Observations of the White Spot, 1933 August 3-September 16.”, MNRAS, 94, 86 (1933)
16. Sheenhan, W., “Planets and Perception : Telescopic Views and Interpretation.” Arizona Press (1988)
17. Williams, A.S., “On the Rotation of Saturn.”, MNRAS, 54, 297 (1894)

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Last Update : 29th September 2012

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